[DOCS] Add docs versioning comment action

[leemthompo]

Adds a friendly automated reminder on PRs that modify documentation files to help enforce our cumulative documentation guidelines.

• Triggers when a PR is opened or reopened that modifies .md files in the docs/ directory
• Posts a single comment explaining cumulative docs and applies_to tagging requirements
• Provides collapsible details with quick do's and don'ts for version tagging
• Never comments more than once on the same PR (checks for existing comments first)
• Includes links to full documentation and where to get help

As we've transitioned to cumulative documentation where all 9.x versions share a single page, contributors need clear guidance on properly tagging version-specific content rather than replacing information for older versions.

Uses github-script to:

1. Filter PRs containing .md file changes in docs/ directory
2. Check for existing bot comments to prevent duplicates
3. Post a standardized reminder with expandable details section

Preview

The comment will look like this when rendered:

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

When to use applies_to tags:

✅ At the page level to indicate which products/deployments the content applies to (mandatory)
✅ When features change state (e.g. preview, ga) in a specific version
✅ When availability differs across deployments and environments

What NOT to do:

❌ Don't remove or replace information that applies to an older version
❌ Don't add new information that applies to a specific version without an applies_to tag
❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
• Reach out in the #docs Slack channel

[leemthompo]

closing in favor of elastic/docs-builder#1667